1. Field of the Invention
The invention pertains to an optical assembly comprising: at least one optical element movable in at least two degrees of freedom and at least one actuator for adjusting the least one optical element; at least one sensor, especially an encoder or another incremental measuring system, for sensing the position of the at least one element in at least two degrees of freedom, and equivalents of such.
2. Description of Related Art
For reflective optical assemblies such as the projection optics used for EUV (extreme ultra-violet) lithography high optical specifications demand that the optical elements are stable to each other to sub-nanometer levels. This requirement is coupled with the necessity of handling an amount of heat absorbed in the optical elements, making a passive system of actuators to manipulate the optical assembly extremely difficult to implement. For example, the relative positions of two mirrors 1 m apart on an Invar™ structure would already change 1 nm at 1 mK temperature change. This is in addition to any long-term dimensional changes during the lifetime of the assembly (typically ten years), such as stress relaxation in the materials, and in particular at joints employed in the design. Also all varieties of oscillatory motions add on.
Such long term effects can, of course, be compensated if one or more optical elements can be adjusted in situ. The actuation of multiple optical elements in multiple degrees of freedom during service has long been applied in refractive optics. Furthermore, with the ability to measure optical wave-front qualities emerging from optics not only manufacturing errors can be compensated to a large extent, but more important long term thermal effects in service.
A number of measurement systems already exist which can offer measurement accuracy and resolution to sub-nanometer level. The most common of these are laser interferometers and capacitor sensors. A laser interferometer is essentially an incremental relative measurement system capable of registering changes in positions from an initial state, and offers high measurement bandwidth. A capacitor is an absolute sensor which has a very limited range, requires close proximity of the sensing surfaces, and has limited bandwidth.
For an active optical positioning system, i. e., a system which permits changes of its position, and which is applicable in EUV or other precision optics, a combination of both of the above is required. This is because the high active positioning assembly demands a very high servo bandwidth which is beyond the capability of most capacitive sensing systems. While the interferometer can fulfill such requirements, it requires the optical elements to be in such an initially ideal state that the image quality of the assembly is already good enough during start-up, or at least in the capture range of any image quality sensing means with which the optics can eventually be brought into the ideal positions. This implies that some means of holding positions of optical elements to nanometer level has to be provided for during power off, which in turn greatly limits the choice of actuation concepts.
From EP 1 182 509 A2 a lithographic projection apparatus is known which comprises a displacement measuring system.
U.S. Pat. No. 6,549,270 B1 relates to an exposure apparatus in which information such as the position, inclination, or shape of a plurality of mirrors constituting a projection optical system is measured with a mirror monitor mechanism.
EP 1 124 161 A2 pertains to a lithographic projection apparatus having a temperature controlled heat shield. Also EP 1 278 089 A2 describes a lithographic apparatus in which the projection system comprises a plurality of optical elements or sensors mounted on a frame.
According to US 2003/10902 A an optical system with a plurality of optical elements is provided which comprises a load receiving portion which takes over the forces exerted by the optical elements.
From U.S. Pat. No. 5,488,514 a mounting seat for an optical element is known which is decoupled from a surrounding annulus that fixes the mount in place in an optical system.
EP 1 312 965 pertains to an optical element holding apparatus.
In US 2001/00358500 A1 inter alia a holding device for an optical element is shown. All cited references are incorporated by reference. Their description here is not complete and does not constitute any admittance of prior art.